dmaengine: pl330: remove set but unused variable

Compiler complains (with W=1):
drivers/dma/pl330.c: In function ‘pl330_release_channel’:
drivers/dma/pl330.c:1782:21: warning:
	variable ‘pl330’ set but not used [-Wunused-but-set-variable]
  struct pl330_dmac *pl330;
                     ^~~~~

Remove the pl330 variable in pl330_release_channel as it is set but
never used.

Signed-off-by: Vinod Koul <vkoul@kernel.org>